ASN Blocking

An Autonomous System Number is a globally unique identifier assigned to a network or group of networks that share a single, clearly defined routing policy. ASNs are managed by IANA and distributed through Regional Internet Registries. When you block an ASN, you are effectively blocking every IP address announced by that entity's BGP (Border Gateway Protocol) routes. This includes all virtual machines, containers, and proxy exits operated by that provider.

The core logic of ASN blocking relies on distinguishing between hosting provider ASNs and consumer ISP ASNs:

• Datacenter ASNs: Owned by cloud providers (AWS AS16509, Google Cloud AS396982, DigitalOcean AS14061). Traffic from these ranges is almost exclusively automated or proxied.
• Residential ISP ASNs: Owned by Comcast, AT&T, etc. Blocking these would deny service to legitimate human users. Effective ASN blocking targets the former, often using commercial IP-to-ASN databases to maintain accuracy.

ASN blocking is typically implemented at the edge router or CDN layer by dropping packets from specific AS_PATHs. This is often achieved via Remotely Triggered Black Hole (RTBH) filtering or by configuring firewall rules that reference a dynamic list of ASN-derived prefixes. Because the decision is made at Layer 3/4, the connection is terminated before any HTTP headers are parsed, making it highly efficient against volumetric scraping from cloud infrastructure.

The primary weakness of ASN blocking is its inability to stop traffic routed through residential IP proxy networks. Sophisticated scrapers bypass datacenter ASN blocks by routing requests through compromised consumer devices or 'proxyware' apps that lease residential IPs. Since these IPs belong to legitimate ISP ASNs, blocking them would cause unacceptable collateral damage. This necessitates a defense-in-depth strategy combining ASN blocking with TLS fingerprinting and behavioral analysis.

In modern edge bot management platforms, ASN blocking is a low-latency signal fed into a bot score heuristic. Rather than a binary block, a request from a high-risk hosting ASN might be assigned a higher suspicion score, triggering a JavaScript challenge or proof-of-work test. This graduated response avoids the brittleness of static blocklists while still leveraging the network-layer intelligence. Providers like Cloudflare and Fastly maintain proprietary ASN reputation databases for this purpose.

Aggressive ASN blocking can inadvertently deny access to legitimate services. Examples include:

• VPN users who exit through cloud provider IPs.
• Academic researchers running scripts from university high-performance computing clusters.
• API integrations from legitimate SaaS platforms hosted on AWS or Azure. To mitigate this, mature implementations use allowlists for specific IPs or authenticated sessions that override the ASN-level block, ensuring business continuity for known partners.

A dynamic trust score assigned to an IP address based on historical behavior, threat intelligence feeds, and association with malicious activity. ASN blocking is often the coarse-grained first pass, while IP reputation provides granular scoring within an ASN.

• Integrates with Real-time Blackhole Lists (RBLs)
• Scores range from -100 (malicious) to +100 (trusted partner)
• Updated continuously via threat intelligence platforms

The process of cross-referencing connecting IP addresses against commercial databases of cloud provider and hosting service ranges. This is the foundational lookup that enables ASN blocking.

• Maps IPs to providers like AWS, Azure, GCP, and DigitalOcean
• Uses WHOIS and BGP routing data
• Identifies traffic from virtual private servers rather than residential or enterprise networks

A security service deployed at the content delivery network edge that uses machine learning and fingerprinting to detect, categorize, and mitigate automated traffic. ASN blocking is one rule in its policy engine.

• Combines JA4 fingerprinting, behavioral analysis, and IP reputation
• Executes JavaScript challenges and Proof-of-Work tests
• Providers: Cloudflare Bot Management, AWS Shield Advanced, Akamai Bot Manager

A network interrogation technique that resolves an IP address back to its hostname, enabling verification of whether traffic originates from a legitimate residential ISP or a cloud data center.

• PTR record query via dig -x <IP>
• Cloud IPs often resolve to generic patterns like *.compute.amazonaws.com
• Used to validate Forward Confirmed Reverse DNS (FCrDNS)

A traffic control strategy that restricts the number of requests a specific IP address or session token can make within a defined time window. Complements ASN blocking by throttling rather than outright denying.

• Token bucket and leaky bucket algorithms
• Headers: X-RateLimit-Remaining, Retry-After
• Prevents resource exhaustion even from non-blocked ASNs

A network routing service that channels bot traffic through IP addresses assigned by consumer ISPs to real home users. This is the primary evasion technique against ASN blocking, making traffic appear as legitimate organic human browsing.

• Providers: Bright Data, Oxylabs, IPRoyal
• Exploits peer-to-peer proxy networks and SDK embedding
• Defeated only by combining ASN blocking with browser integrity checks and behavioral analysis